This invention relates to radar systems, and more particularly to monopulse radar systems.
Monopulse radar systems are widely used for radar surveillance and tracking, and for missile tracking or homing systems.
Monopulse radar systems are advantageous by comparison with the use of single-function surveillance radars combined with altitude-determining radar systems, because a single radar system provides the information required not only to establish the presence of a target in surveillance operation, but also provides the information required to determine both azimuth and elevation angle of the target relative to the boresight of the sum beam. It should be noted that the terms xe2x80x9cazimuthxe2x80x9d and xe2x80x9celevationxe2x80x9d are conventional terms used to designate two orthogonal directions, which are not necessarily associated with true azimuth or true elevation.
FIG. 1 is a simplified block diagram of a prior-art monopulse antenna system using an array antenna. In FIG. 1, the monopulse radar receiving system 10 includes a set 12 of individual antenna elements 121, 122, . . . 12N. The individual receive antenna elements 121, 122, . . . 12N are formed into a receive array having two dimensions. The individual antenna elements 121, 122, . . . 12N receive signals reflected from a target, and couple the received signals r1, r2, . . . , rN to various input ports 14i1, 14i2, . . . , 14iN of an analog beamformer 14. Beamformer 14 processes the signals as generally described in conjunction with FIG. 2 to produce analog sum (xcexa3) signals, azimuth difference signals (xcex94A) and elevation difference signals (xcex94E). The analog sum signals represent the summation of all the signals received by the array of individual antenna elements. The analog azimuth difference signals represent the difference between the signals received by the antenna elements of the right and left halves of the array of antenna elements, while the analog elevation difference signals represent the difference between the signals received by the antenna elements of the upper and lower halves of the array. The analog sum signals are applied from beamformer 14 to a radio-frequency (RF) receiver illustrated as a block 16xcexa3 of a xcexa3 processing channel, which performs standard analog receiver functions such as low-noise amplification and or downconversion to an intermediate frequency (IF). The received analog signals from receiver 16xcexa3 are applied to an IF receiver 18xcexa3 which performs further standard functions such as IF amplification and detection, to produce baseband analog signals. The baseband signals from IF receiver 18xcexa3 are applied to an analog-to-digital converter (ADC) 20xcexa3, which converts the analog signals into quantized or digital signals representing the signals received by the sum channel. The digital signals from analog-to-digital converter 20xcexa3 are applied to conventional sum-channel waveform digital processing illustrated as a block 22xcexa3, which produces the processed sum-channel signal for evaluation by a conventional threshold or other detector 24, which evaluates for the presence of absence of a target in the receive sum beam.
The analog azimuth difference signals xcex94A of FIG. 1 are applied from beamformer 14 to a radio-frequency (RF) receiver illustrated as a block 16xcex94A of a xcex94A processing channel, which performs standard analog receiver functions. The received analog signals from receiver 16xcex94A are applied to an IF receiver 18xcex94A which performs further standard functions such as IF amplification and detection, to produce baseband analog signals for the xcex94A channel. The baseband signals from IF receiver 18xcex94A are applied to an analog-to-digital converter (ADC) 20xcex94A, which converts the analog signals into quantized or digital signals representing the signals received by the azimuth difference channel. The digital signals from analog-to-digital converter 20xcex94A are applied to conventional azimuth-difference-channel waveform digital processing illustrated as a block 22xcex94A, which produces the processed azimuth-difference signal for evaluation by a conventional azimuth monopulse ratio detector 26, which evaluates the ratio of the azimuth difference signal to the sum signal to determine the azimuth angle of the target relative to boresight.
The analog elevation difference signals xcex94E of FIG. 1 are applied from beamformer 14 to a radio-frequency (RF) receiver illustrated as a block 16xcex94E of a xcex94E processing channel, which performs standard analog receiver functions. The received analog signals from receiver 16xcex94E are applied to an IF receiver 18xcex94E which performs further standard functions such as IF amplification and detection, to thereby produce baseband analog signals for the xcex94E channel. The baseband signals from IF receiver 18xcex94E are applied to an analog-to-digital converter 20xcex94E, which converts the analog signals into quantized or digital signals representing the signals received by the elevation difference channel. The digital signals from analog-to-digital converter 20xcex94E are applied to conventional azimuth-difference-channel waveform digital processing illustrated as a block 22xcex94E, which produces the processed elevation-difference signal for evaluation by a conventional elevation monopulse ratio detector 30, which evaluates the ratio of the elevation difference signal to the sum signal to determine the elevation angle of the target relative to boresight.
In the arrangement of FIG. 1, the RF receiver blocks and the IF receiver blocks are ordinarily assumed to introduce no perturbation of the received signals, so that the analog signals at the output of the beamformer and the digital signals at the outputs of the analog-to-digital converters can be deemed to be the same, although represented in different form. In the conventional arrangement of FIG. 1, the beamformed signal (either at the beamformer outputs or at the ADC outputs) can be expressed as                     Σ        =                              ∑                          k              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    w              ∑                        ⁢                          xe2x80x83                        ⁢                          (              k              )                        ⁢                          xe2x80x83                        ⁢            r            ⁢                          xe2x80x83                        ⁢                          (              k              )                                                  1                                    Δ          A                =                              ∑                          k              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    w                              Δ                A                                      ⁢                          xe2x80x83                        ⁢                          (              k              )                        ⁢                          xe2x80x83                        ⁢            r            ⁢                          xe2x80x83                        ⁢                          (              k              )                                                  2                                    Δ          E                =                              ∑                          k              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    w                              Δ                ⁢                                  xe2x80x83                                ⁢                E                                      ⁢                          xe2x80x83                        ⁢                          (              k              )                        ⁢                          xe2x80x83                        ⁢            r            ⁢                          xe2x80x83                        ⁢                          (              k              )                                                  3      
where wxcexa3, wxcex94A, and wxcex94E are the sum, azimuth-difference and elevation-difference beamforming weights, and {r(k)} are the received signals at each antenna element of the array.
As mentioned, the target detection in block 24 of FIG. 1 is conventional, and amounts to some type of thresholding. When a target is identified by block 24, the azimuth and elevation angles of the target, mA and mE, are determined from a monopulse table look-up                               m          A                =                  Re          ⁢                      xe2x80x83                    ⁢                      (                                          Δ                A                            ∑                        )                                      4                                    m          E                =                  Re          ⁢                      xe2x80x83                    ⁢                      (                                          Δ                A                            ∑                        )                                      5      
The corresponding antenna patterns for the sum, azimuth and elevation beams are given by                                                         g              ∑                        ⁢                          xe2x80x83                        ⁢                          (                                                T                  x                                ,                                  T                  y                                            )                                =                                    ∑                              k                =                1                            N                        ⁢                          xe2x80x83                        ⁢                                          w                ∑                            ⁢                              xe2x80x83                            ⁢                              (                k                )                            ⁢                              xe2x80x83                            ⁢              exp              ⁢                              xe2x80x83                            ⁢                              (                                  ⅈ                  ⁢                                      xe2x80x83                                    ⁢                  2                  ⁢                                      xe2x80x83                                    ⁢                                      π                    λ                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                                            T                          x                                                ⁢                                                  x                          k                                                                    +                                                                        T                          y                                                ⁢                                                  y                          k                                                                                      )                                                  )                                                    ⁢                  xe2x80x83                            6                                                g                          Δ              ⁢                              xe2x80x83                            ⁢              A                                ⁢                      xe2x80x83                    ⁢                      (                                          T                x                            ,                              T                y                                      )                          =                              ∑                          k              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    w                              Δ                ⁢                                  xe2x80x83                                ⁢                A                                      ⁢                          xe2x80x83                        ⁢                          (              k              )                        ⁢                          xe2x80x83                        ⁢            exp            ⁢                          xe2x80x83                        ⁢                          (                              ⅈ                ⁢                                  xe2x80x83                                ⁢                2                ⁢                                  xe2x80x83                                ⁢                                  π                  λ                                ⁢                                  xe2x80x83                                ⁢                                  (                                                                                    T                        x                                            ⁢                                              x                        k                                                              +                                                                  T                        x                                            ⁢                                              y                        k                                                                              )                                            )                                                  7                                                g                          Δ              ⁢                              xe2x80x83                            ⁢              E                                ⁢                      xe2x80x83                    ⁢                      (                                          T                x                            ,                              T                y                                      )                          =                              ∑                          k              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    w                              Δ                ⁢                                  xe2x80x83                                ⁢                E                                      ⁢                          xe2x80x83                        ⁢                          (              k              )                        ⁢                          xe2x80x83                        ⁢            exp            ⁢                          xe2x80x83                        ⁢                          (                              ⅈ                ⁢                                  xe2x80x83                                ⁢                2                ⁢                                  xe2x80x83                                ⁢                                  π                  λ                                ⁢                                  xe2x80x83                                ⁢                                  (                                                                                    T                        x                                            ⁢                                              x                        k                                                              +                                                                  T                        y                                            ⁢                                              y                        k                                                                              )                                            )                                                  8      
where (Tx, Ty) are the direction cosines and (Xk, Yk) are the antenna element locations.
The adoption of high-speed digital signal processing allowed the morphology or topology of the xe2x80x9cprincipally-analogxe2x80x9d monopulse system of FIG. 1 to be adapted for digital beamforming, as illustrated in the simplified block diagram of FIG. 2. In FIG. 2, elements corresponding to those of FIG. 1 are designated by like reference numerals, but in the 200 series. In FIG. 2, the receive portion of the radar system 200 includes set 12 of N antenna elements 121, 122, . . . , 12N as in the case of FIG. 1. The corresponding received signals r1, r2, . . . , rN are coupled from each receive antenna element of set 12 to a corresponding RF receiver 2161, 2162, . . . , 216N, where the signals are low-noise amplified, filtered, and converted to IF frequency. From RF receivers 2161, 2162, . . . , 216N, the analog received signals are coupled to a corresponding set 218 of IF-receivers, including IF receivers 2181, 2182, . . . , 218N. The IF receivers of set 218 amplify and possibly otherwise process the IF-frequency signals, to produce signals at baseband. The baseband signals from IF amplifier set 218 are applied to corresponding analog-to-digital converters (ADCs) 2201, 2202, . . . , 220N of a set 220 of ADCs. The digital signals representing the N received signals are applied from the N ADCs of set 220 to the N input ports 214i1, 214i2, . . . , 214iN of digital beamformer 214. Beamformer 214 processes the signals represented by the digital numbers in essentially the same way as the beamformer 214 of FIG. 1, except that beamformer 214 does the processing in digital form, whereas the beamformer of FIG. 1 is an analog apparatus. Beamformer 214 produces digital sum (xcexa3) signals, azimuth difference signals (xcex94A) and elevation difference signals (xcex94E). The digital sum signals represent the summation of all the signals received by the array of individual antenna elements, and the digital azimuth difference signals represent the difference between the signals received by the antenna elements of the right and left halves of the array of antenna elements, while the digital elevation difference signals represent the difference between the signals received by the antenna elements of the upper and lower halves of the array, all as in the case of the analog beamformer of FIG. 1.
From the (xcexa3), (xcex94A), and (xcex94E) output ports of beamformer 214 of FIG. 2, the digital (xcexa3), (xcex94A), and (xcex94E) signals are applied to processing which corresponds to that of FIG. 1, namely the xcexa3 signals are applied to a waveform processing block 22xcexa3 and thence to a detection block 24, the xcex94A signals are applied to a waveform processing block 22xcex94A, and the xcex94E signals are applied to another waveform processing block 22xcex94E. Blocks 26 and 30 perform the same functions as that performed in the arrangement of FIG. 1, namely the taking of the ratio of the sum signal xcexa3 to the azimuth difference signal xcex94A and to the elevation difference signal xcex94E, respectively, and from those ratios, looking up the target angle.
FIG. 3 is a simplified representation of the processing which is performed by the analog beamformer 14 of FIG. 1 or by the digital beamformer 214 of FIG. 2. In FIG. 3, beamformer 14, 214 receives r1 signals (from the first antenna element of the array, not illustrated) and couples the signals to three multipliers 310xcexa31, 310A1, and 310E1. The r2 received signals are applied to three multipliers 310xcexa32, 310A2, and 310E2, and the rN received signals are applied to three multipliers 310xcexa3N, 310AN, and 310EN. Multipliers 310xcexa31, 310xcexa32, and 310xcexa3N are associated with the xcexa3 beam of the system, multipliers 310A1, 310A2, and 310AN are associated with the xcex94A beam, and multipliers 310E1, 310E2, and 310EN are associated with the xcex94E beam. Each multiplier also receives a weight for weighting the return signals flowing through the multiplier. More particularly, multipliers 310xcexa31, 310xcexa32, . . . , and 310xcexa3N are associated with weights W1xcexa3, W2xcexa3, . . . , and WNxcexa3, respectively; multipliers 310A1, 310A2, . . . , and 310AN are associated with weights W1xcex94A, W2xcex94A, . . . , and WNxcex94A, respectively, and multipliers 310E1, 310E2, . . . , and 310EN are associated with weights W1xcex94E, W2xcex94E, . . . , and WNxcex94E, respectively. The multipliers multiply the r1, r2, . . . , rN signals by the various weights to produce signals which are summed. The weighted signals produced by multipliers 310xcexa31, 310xcexa32, . . . , and 310xcexa3N are summed by a summing circuit 312xcexa3 to produce the xcexa3 signal at the output of the beamformer 14, 214, the weighted signals produced by multipliers 310A1, 310A2, . . , and 310AN are summed together by a summing circuit 316A to produce the xcex94A signal, and the weighted signals produced by multipliers 310A1, 310A2, . . . , and 310AN are summed together by a summing circuit 312E to produce the xcex94E signal.
Improved monopulse systems are desired.
A radar return signal detection system according to an aspect of the invention is for determining the presence of a target and for determining the azimuth and elevation angles of arrival of the return signal from the target according to an aspect of the invention includes an array of receiving antenna elements for receiving the return signal. This array is preferably a two-dimensional array of elemental antennas. The system also includes a combination of (a) analog-to-digital conversion means and (b) a beamformer. The combination is coupled to each of the antenna elements for receiving signals representative of the return signal. The combination also includes two beamforming ports at which digital first and second signals are produced, with the first signal representing a sum beam and the second signal representing a difference beam. The second signal is in the form of a complex number in which the azimuth difference and the elevation difference information are encoded. The system also includes digital sum signal processing means coupled to the combination for receiving the first signal, and for generating a sum signal indication for determining the presence or absence of a target. A digital difference signal processing means is coupled to the combination for receiving the complex number, and processing the first signal with the complex number to produce the azimuth and elevation angles. In a particularly advantageous embodiment of the invention, the complex number is divided by the sum signal to produce a further complex number in which the real component corresponds to the azimuth monopulse ratio, and in which the imaginary component corresponds to the elevation monopulse ratio.
In one embodiment of the invention, the beamformer is an analog beamformer for receiving analog signals from the antenna elements and for producing the first and second signals in analog form, and the analog-to-digital conversion means comprises first and second analog-to-digital converters coupled to the beam output of the analog beamformer for converting the analog first and second signals into digital form.
In another embodiment of the invention, the analog-to-digital conversion means comprises a plurality of analog-to-digital converters equal in number to the number of the receiving antenna elements in the array, with each of the analog-to-digital converters coupled to one of the receiving antenna elements, for converting analog signals received by each of the receiving antenna elements into digital form. In this embodiment, the beamformer is a digital beamformer coupled to the plurality of analog-to-digital converters, for generating the digital first and second signals from the digital signals produced by the analog-to-digital converters.